A variety of methods for confinement of light and for localization and enhancement of electromagnetic field in nanostructures, for the purpose of enhancing various localized linear and nonlinear optical phenomena are known in the prior art (See, for example, A. Wokaun, 1984: M. Moskovits, 1985). Most attention in the prior art has been related to the phenomena of Surface Enhanced Raman Scattering (SERS), based on localization and confinement of light near the surfaces of substrates with nanoscale structure. SERS has proven to be a powerful analytical tool for ultra sensitive chemical and biochemical analysis (K. Kneipp et al., 1999).
One SERS-based structure that has been proposed employs an optical structure composed of a metal island film (MIF) over a smooth metal surface (H.-G. Binger et al., 1995, G. Bauer et al., 2003). A metal island film consists of a random two-dimensional array of metal particles, each of several (typically, 2-10) nm in largest size dimension. The shapes of the metal particles are also variable, so it is difficult to characterization the particles structurally. (The particles form a stochastic array of particles resembling oblate spheroids with all minor axis oriented normal to substrate surface, e.g., glass, quartz, or silicon.) For a variety of reasons that will become clear below.
The metal island film MIF is separated from a smooth metal layer by an intermediate spacer layer made from optically transparent dielectrical material, the thickness of which controls the strength of the interaction between the plasmons localized on the islands and the surface plasmons of smooth metal layer. The metal particles (islands) can be thought of as nanoscopic antennas, collecting the incident radiation and then transferring the energy into the nearby gap modes, that may be trapped into guided modes propagating in all directions in plane of surface (omnidirectional coupling). The ability of structure to absorb light at specific wavelength depends on the existence of an optimal spacer layer thickness that will maximize absorption in structure for specific wavelength close to that of excitation light (Leitner et al., Appl Opt 1993; W. R. Holland et al., 1983, T. Kune et al., 1995). For a variety of reasons that will become clear below, the maximum enhancement achievable with such MIF structures is limited to between about 106-108.
The phenomenon of interaction of localized plasmons (LP) with surface plasmon polaritons (SPP) in plasmon materials has been discovered and new method of excitation of SPP in plasmon resonant smooth films mediated by nanoparticles has been proposed (S. Hayashi et al., 1996). An interesting phenomenon associated with SPP excitation is the generation of a strong electromagnetic field near the metallic surface. It is a generally accepted mechanism that a strong electromagnetic field leads to enhancement of various linear and nonlinear optical processes near the surface via a mechanism of surface-enhanced spectroscopy (M. Moskovits, 1985; G. C. Schatz and R. P. Van Duyne, 2002). According to this mechanism, the enhancement of SERS signal is proportional to E4, where E is electromagnetic field near metal surface.
One typical application of this phenomenon is the surface enhanced Raman scattering of molecules adsorbed on metallic surfaces that support plasmon resonances at both the excitation and scattering wavelengths. Typical enhancement achieved by using electrolysis roughened silver or by using substrate prepared by nanosphere lithography (J. C. Hulteen et al., 1999) is in the range 106-108. In general, the degree of enhancement seen is not uniform across the sensor nor reproducible.
The inability to control parameters of MIF metal surface and intrinsic limitations in size of metal particles to less than 5 nm (V. Matyushin, A et al., 2004) precludes their use for SERS (H.-G. Binger et al., 1995) limits the sensitivity of such a system since MIF-metal substrate structures do not have strong enhancement of Raman signal. Therefore MIF-metal substrate have been reduced to practice only for enhancement of fluorescence in so called “resonant nanocluster biochip” technology (G. Bauer et al., 2003; T. Schalkhammer et al., 2003).